This invention is concerned with high performance copper alloy and its manufacturing methods for electrical and electronic parts such as leadframe or connector materials which have good electrical(thermal) conductivity, superior mechanical properties, and high thermal stability of tin-lead plating. Copper-base alloys are widely used for electrical and electronic structural parts requiring high electrical conductivity and mechanical strength such as semiconductor leadframe and high strength electric materials such as connectors. Informations of general properties of semiconductor leadframe design and materials are described detailed in a book Microelectronics Packaging Handbook ed. by R. Tummala, Van Nostrand, 1989 New York, and in a Product Application Report of DESIGNING AND ADVANCED COPPER-ALLOY LEADFRAME MATERIAL by Y.G. Kim in SEMICONDUCTOR INTERNATIONAL April 1985, A CHANERS PUBLICATION, Denver, Colo. 80206. Leadframe is the backbone of a molded IC packaging. Fabricated by stamping a strip of sheet metal or by chemical etching, it serves as a holding fixture during the assembly process, then after molding, becomes a part of the IC package. After the molding process, legs of leadframe are electroplated by tin/lead for solderability and surface stability. Nowdays, the thermal stability of plated parts is becoming increasingly important, for some section of current semiconductor parts is used at high temperature at above 100.degree. C. Therefore, during their uses at elevated temperatures, it is required to minimize any harmful interfacial reactions between the copper-base alloy and the electroplated tin/lead layer. When brittle intermetallic phases are formed at the interface between base metal and plating, the coated layer can be peeled off from the base metals or the coated layer can be cracked during with increasing temperature due to the increased diffusion of metallic elements to the interface. So the thermal stability of coated layer becomes important and it is a critical parameter in a selection of material. For a given coating material, formation of harmful intermetallic phases at the interface depend on chemical composition (alloying elements) in the base copper alloy matrix. Although the formation of the brittle intermetallic phases are mainly controlled by the diffusion of the alloying elements, it will be accelerated if the amount of the residual stress induced by the cold rolling is high. Therefore, it is necessary to remove or minimize the amount of residual stress by thermal annealing after the final rolling step.
It is required for semiconductor leadframe or connector materials to have high electrical and thermal conductivity, high thermal softening resistance, good electroplatability and good solderability. Specially, because of the automation of semiconductor packaging process, the need of high strength is increasing. But, generally, as the strength of material increase, the electrical conductivity and its elongation go down. High electrical conductivity is important to increase the rate of heat dissipation when the chip works. The ability of heat dissipation from the chip to the printed circuit board depends on the thermal conductivity of leadframe materials. The high tensile elongation is also required both to increase formability and to improve the resistance to the lead bend fatigue failure.
In conventional semiconductor leadframe materials, there are Cu-base alloys and Fe-base alloy to gain these properties. In Cu-base alloys, there are CDA 19400 (U.S. Pat. No. 3,522,039) and CDA 19500 invented by U.S. OLIN Company, and Cu-Ni-Si-P alloys (U.S. Pat. No. 4,466,939) by Poongsan Corp. In Korea etc.
CDA 19400 has the inferior strength properties than the alloy of U.S. Pat. No. 4,466,939. CDA 19500 has inferior elongation, and is also expensive because it contains the 0.8% cobalt of high price element. In the Fe-base alloy, Fe-42Ni alloy is being used. This alloy has high-strength, but has the inferior heat dissipation property because electrical conductivity is IACS 2-3% which is 1/20 of that of the copper-base alloys. And, other copper-base alloys available have been patented, such as U.S. Pat. No. 4,594,221 and U.S. Pat. No. 4,728,372. However, the multipurpose copper alloys covered by those alloys fails to meet the desired goals of high tensile strength, good electrical (thermal) conductivity, good thermal stability of Sn/Pb plating, and good elongation. Therefore, it is highly desirable to have new leadframe alloys having a tensile strength of about 80 ksi (57kg/mm) while maintaining an electrical conductivity of about 55% IACS or higher, and tensile elongation of about 6% or higher, while exhibiting thermal stability for the tin/lead coating at elevated temperature exposure. And, thermal softening resistance exceeding the temperature of 400.degree. C. is required in the developing new Bare Bonding Technology which is an innovative processing method of Semiconductor IC packaging (Si chips are directly bonded to the Cu-leadframe without silver plating, and Cu-wire is used instead of gold wire). Thus, the mechanical properties which have high strength both at ambient and elevated temperature is required. And high elongation with a good electrical (thermal) conductivity is also desired for the new surface mounting technology. In the surface mounting technology, lead configurations require multiple bending. When the elongation is not high enough, crackings at the bend radii occur. Although the tensile strength of conventional copper alloys is lower than the of Fe-42Ni alloys, the copper alloy leadframes are widely used for plastic molded IC packaging because of its significantly higher electrical conductivity, compared with that of the Fe-42Ni alloy.
The inventor of the present invention obtained British Pat. No. 2,158,095, Japanese Pat. No. 1,426,889, and Korean Pat. No. 18,126 for the corresponding U.S. Pat. No. 4,466,939. The alloy of U.S. Pat. No. 4,466,939 comprised of 0.05-3.0% by weight nickel, 0.01-1.0% by weight silicon and 0.01-0.1% by weight phosphorus with balanced copper. The alloy satisfied the desired requirements for mechanical and physical properties specified in this invention, which is being now used commercially in semiconductor industries for good service at ambient temperature after the tin/lead plating on the legs of leadframe. The U.S. Pat. No. 4,466,939 is strengthened by precipitation hardening and cold work hardening during the thermomechanical processing. The precipitates formed were Ni.sub.2 Si and Ni.sub.3 P in the coppermatrix. It is ideal to have the exact amount of nickel, silicon and phosphorus in the copper matrix. However, if there exist any excess amount of silicon or phosphorus in the matrix, it will be present as free silicon or phosphorus in the matrix after forming Ni.sub.2 Si and Ni.sub.3 P precipitates. Then, the free silicon and phosphorus will diffuse out to the interface between the copper alloy matrix and the tin or Sn/Pb plated layer. At elevated temperature exposure, the diffusion will be accelerated, and then the formation of brittle intermetallic compounds comprising silicon, phosphorus, tin and lead can occur at the interface.
Therefore, the tin or the tin/lead layer can be peeled off from the copper-matrix, and cracking at the bent region could occur. Furthermore, the separation or peeling off the coated layer from the base matrix can also be affected by the residual stresses induced by the final cold rolling during the manufacturing stress by annealing heat treatment. In order to maintain the thermal stability of the tin/lead coated layer for the copper leadframe alloys, the presences of free silicon and phosphorus in the copper matrix should be avoided to prevent the formation of intermetallic compounds at the interface. To eliminate this problem, exact stoichiometric ratio of nickel, silicon and phosphorus is required, but it is practically impossible in commercial melting and casting stages. In other words, there are always some loss of alloying elements, therefore it is almost impossible to have an exact amount of the desired stoichiometric ratio.